Various types of multi-stage compressors for use in an air conditioning apparatus have been proposed. One such known multi-stage compressor has a configuration as described below. A lower stage rotary compression mechanism is installed below an electric motor installed at a central portion of a closed housing. The compressed gas is discharged into the closed housing, and this intermediate pressure gas is sucked into a higher stage scroll compression mechanism installed above the electric motor. Thereby, two-stage compression is performed. (For example, see Patent Citation 1.)
Patent Citation 2 proposes a multi-stage compressor as set forth below. An electric motor, a lower stage rotary compression mechanism and a higher stage rotary compression mechanism are installed in a closed housing. An intermediate pressure gas compressed by the lower stage rotary compression mechanism is discharged into a second closed chamber provided in the closed housing, and at the same time, the intermediate pressure gas extracted from a refrigerant circuit side is injected into the second closed chamber, so that this intermediate pressure injection gas and the intermediate pressure gas compressed by the lower stage rotary compression mechanism are mixed together. The mixed gas is sucked by the higher stage rotary compression mechanism, whereby two-stage compression is carried out.
In the multi-stage compressors disclosed in Patent Citations 1 and 2, the interior of the closed housing is made an intermediate pressure chamber, or a portion of the closed housing is partitioned to form an intermediate pressure chamber. The intermediate pressure refrigerant compressed by the lower stage compression mechanism is discharged into the intermediate pressure chamber, and at the same time, the intermediate pressure refrigerant extracted from the refrigerant circuit is injected into the intermediate pressure chamber. This intermediate pressure refrigerant is sucked by the higher stage compression mechanism, so that two-stage compression is performed.    Patent Citation 1: Japanese Unexamined Patent Application, Publication No. 5-87074    Patent Citation 2: Japanese Unexamined Patent Application, Publication No. 2000-54975